In the latter half of the twentieth century, there began a phenomenon known as the information revolution. While the information revolution is a historical development broader in scope than any one event or machine, no single device has come to represent the information revolution more than the digital electronic computer. The development of computer systems has surely been a revolution. Each year, computer systems become faster, store more data, and provide more applications to their users.
A modern computer system typically comprises a central processing unit (CPU) and supporting hardware necessary to store, retrieve and transfer information, such as communications buses and memory. It also includes hardware necessary to communicate with the outside world, such as input/output controllers or storage controllers, and devices attached thereto such as keyboards, monitors, tape drives, disk drives, communication lines coupled to a network, etc. The CPU is the heart of the system. It executes the instructions which comprise a computer program and directs the operation of the other system components.
From the standpoint of the computer's hardware, most systems operate in fundamentally the same manner. Processors are capable of performing a limited set of very simple operations, such as arithmetic, logical comparisons, and movement of data from one location to another. But each operation is performed very quickly. Programs which direct a computer to perform massive numbers of these simple operations give the illusion that the computer is doing something sophisticated. What is perceived by the user as a new or improved capability of a computer system is made possible by performing essentially the same set of very simple operations, but doing it much faster. Therefore continuing improvements to computer systems require that these systems be made ever faster.
The overall speed of a computer system (also called the throughput) may be crudely measured as the number of operations performed per unit of time. There are numerous ways in which system speed might be improved, but conceptually the simplest and most fundamental of all improvements is to increase the speed at which the basic circuits operate, i.e., to increase the clock speeds of the various components, and particularly the clock speed of the processor(s). E.g., if everything runs twice as fast but otherwise works in exactly the same manner, the system will perform a given task in half the time.
Clock speeds are necessarily limited by various design parameters, and in particular are limited by signal propagation delays. In general, clock speeds can be increased if the length of signal paths is reduced, i.e., by shrinking the size of the logic elements. Early computer processors, which were constructed from many discrete components, were susceptible to significant speed improvements by shrinking component size, reducing discrete component numbers, and eventually, packaging the entire processor as an integrated circuit on a single chip. Modern processor chip designs often include one or more caches on the same integrated circuit chip as the processor, and in some cases include multiple processors on a single integrated circuit chip.
Despite the enormous improvement in speed obtained from integrated circuitry, the demand for ever faster computer systems has continued. With this demand comes a need for even further size reduction in the logic circuitry within an integrated circuit chip.
A typical integrated circuit chip is constructed in multiple layers. Many active and passive elements are formed on a substrate (usually silicon). A dielectric layer is placed over the elements, and multiple conductive layers, each separated by another dielectric layer, are formed over the elements. The conductive layers carry power and ground potentials, as well as numerous signal interconnects running among active elements. Each conductive layer comprises multiple discrete conductors, often running substantially in parallel. Conductive interconnects between conductive layers, or between a conductive layer and an active or passive element, are formed as holes in the dielectric layers, called vias, into which a conductive metal, such as aluminum or copper, is introduced.
The number of active elements in a typical processor dictates a very large number of interconnections, and since these must be packaged within a small area, the size of individual interconnections is limited. Conductors, whether in the conductive layer or the via between conductive layers, have a small, finite resistance, which grows as the cross-sectional area of the conductor shrinks. Increasing the number of logic elements on a chip requires a larger number of conductors, which in turn reduces the amount of space available for each individual conductor. If all other design parameters remain the same, this has the effect of increasing the resistances of the individual conductors. A need exists for improved design techniques for forming interconnection conductors, which will support reduced size of interconnects and greater circuit element density.
Recently, it has been suggested that carbon nanotubes might be used to form conductive pathways in integrated circuits. Carbon nanotubes are pure carbon molecular structures in which a graphite-like structural layer of covalently bonded carbon atoms is wrapped around into a cylindrical shape. Such a structure has a diameter in the nanometer range, and is potentially orders of magnitude longer in the axial dimension. Some carbon nanotubes have extremely high electrical conductivity up to a current limit. The conductivity of these carbon nanotubes is significantly higher (by some estimates, an order of magnitude higher) than that of ordinary metals. Additionally, the current capacity of carbon nanotubes is higher than metals, so that use of nanotubes as conductors can be expected to improve the long-term stability of the form and electrical resistance of the structure.
While the high conductivity of certain carbon nanotubes suggests possible application in electronic circuits, there are significant engineering hurdles involved in design and commercial production of a successful device using carbon nanotubes.